A coal-based direct reduction process used as an alternative to a blast furnace process is a process (so-called carbon composite method) in which iron oxide agglomerates incorporated with a carbonaceous material are heated by radiation in a rotary hearth furnace to produce reduced iron, and this process has already successfully been brought into practical application on a commercial scale. However, this process has the problem that the strength of the agglomerates and heat transmission in the agglomerates are insufficient depending on the properties of the coal used as the reducing agent, thereby influencing the properties of a reduced iron product.
The inventors of the present invention improved the above-mentioned carbon composite method to develop a method in which iron oxide agglomerates incorporated with a carbonaceous material are heated by radiation in a rotary hearth furnace to produce reduced iron, and then the reduced iron is melted by further heating at a high temperature to separate between a metal and a slag and recover the metal. Also, practical application of this method on a commercial scale has been advanced. However, this process has the problem that the metal and the slag cannot be sufficiently separated depending on the properties of the coal used as the reducing agent, and thus the metal remains in the slag, thereby decreasing the recovery yield of the metal.
In the field of conventional nonferrous metallurgy, as a method for producing a titanium oxide-containing slag by separating an iron component from a material, for example, an ilmenite ore containing titanium oxide and iron oxide, a method is used, in which the ilmenite ore is supplied to an electric furnace together with a carbonaceous reducing agent so that iron oxide is reduced, melted, and then taken out as melted iron, and a titanium oxide-containing slag is recovered as an intermediate product for a titanium refining raw material. However, in this method, the temperature in the furnace is decreased by proceeding of reduction reaction of iron oxide, which is endothermic reaction, and thus much electric power is consumed for maintaining the temperature in the furnace. Also, the method has the problem that a large amount of melted FeO is produced in the treatment process, and thus the refractory in the furnace is greatly damaged by the melted FeO. Therefore, it is difficult to effectively produce a titanium oxide-containing slag using an electric furnace. There is further the problem that the inside of the furnace must be maintained in a highly reducing atmosphere for reducing iron oxide, and thus titanium oxide is also reduced by the highly reducing atmosphere.
Accordingly, the inventors of the present invention have advanced research and development for practical application of the above-described method disclosed by the inventors in consideration that as an alternative to the electric furnace method, the method can be basically applied to production of a slag containing an oxidized nonferrous metal, which is an intermediate product for nonferrous metallurgy. This method basically requires no electric power and does not damage the refractory because melted FeO is not produced. However, there still remains the problem that the ash in the coal used is mixed in the slag as in the above-described electric furnace method, thereby decreasing the product's value. There is further the problem that the metal and the slag are not sufficiently separated depending on the properties of the coal used as the reducing agent, and thus metallic iron is mixed in the slag to decrease the content of an oxidized nonferrous metal in the slag, thereby further decreasing the product's value.
On the other hand, an attempt has been made to upgrade low-rank coal such as high-volatile coal by solvent treatment to produce a carbonaceous material for metallurgy. When high-volatile coal without thermal plasticity is treated in a solvent at about 400° C., the coal is separated into an extract with the solvent and a residue. The extract is known to have thermal plasticity which is absent from the original coal. Also, it is indicated that high-strength coke usable for a blast furnace and a cupola can be produced using a mixture of the extract and the original coal or the like.
Since the conventional method for upgrading coal with a solvent is intended to be used in a vertical furnace such as a blast furnace or the like, a carbonaceous material is required to have load strength, and an operation of expressing a caking property other than thermal plasticity must be added. Furthermore, the conventional coal upgrading method uses, as the solvent, a hydrogen donor substance such as tetralin or the like for increasing the coal dissolving power, or a nitrogen-containing solvent such as N-methylpyrrolidinone or coal tar, and thus the method is disadvantageous to industrial production for the following reasons:
Since the hydrogen donor solvent loses its hydrogen donating property in extraction, the solvent must be re-hydrogenated for recycling the solvent. However, hydrogen is very expensive, and there has been found substantially no example of commercial application in the field of metallurgical use. The nitrogen-containing solvent has excessively high compatibility with coal, and thus the solvent and the extracted coal are bonded together, thereby failing to recover the solvent. This causes the problem of failing to recycle the solvent.
The present invention has been achieved in consideration of the above-described problems, and an object of the invention is to provide a method for upgrading low-rank coal such as high-volatile coal, which is unsuitable for the conventional carbon composite method, to produce upgraded coal for metallurgy which is suitable for the carbon composite method. Another object of the present invention is to provide a method for producing a high-quality reduced metal and a slag containing an oxidized nonferrous metal using the upgraded coal for metallurgy.